Chabazite zeolite catalysts having low silica to alumina ratios

ABSTRACT

Disclosed are zeolite catalysts having the CHA crystal structure with a low silica to alumina ratio, as well as articles and systems incorporating the catalysts and methods for their preparation and use. The catalysts can be used to reduce NOx from exhaust gas streams, particularly those emanating from gasoline or diesel engines.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/612,142, filed Nov. 4, 2009, which claims the benefit of U.S.Provisional Application No. 61/111,960, filed on Nov. 6, 2008, theentire contents of each of which are hereby incorporated by reference intheir entireties.

TECHNICAL FIELD

Embodiments of the invention relate to zeolites that have the CHAcrystal structure, methods for their preparation, and catalytic articlesand systems comprising such zeolites. More particularly, embodiments ofthe invention pertain to CHA zeolite catalysts, methods for theirpreparation, and their use in exhaust gas treatment systems.

BACKGROUND

Zeolites are aluminosilicate crystalline materials having rather uniformpore sizes which, depending upon the type of zeolite and the type andamount of cations included in the zeolite lattice, typically range fromabout 3 to 10 Angstroms in diameter. Both synthetic and natural zeolitesand their use in promoting certain reactions, including the selectivereduction of nitrogen oxides with ammonia in the presence of oxygen, arewell known in the art.

One particular zeolite that has found use as a catalyst is chabazite(CHA). Methods for its preparation are known in the art. For example,U.S. Pat. No. 4,544,538 to Zones discloses the synthetic preparation ofhigh silica form (Si/Al ratio of ˜15-30) of chabazite known as SSZ-13.It is prepared using an organically templated(N,N,N-trimethyl-1-adamantammonium) hydrothermal synthesis at hightemperature (˜150° C.) and autogenous pressure.

There is a desire to obtain and/or prepare CHA catalysts via processesother than the organically templated hydrothermal synthesis process usedto prepare the high silica SSZ-13 CHA. In this way, it is possible thatCHA catalysts can be obtained and/or prepared in a more affordable andeconomical manner. In addition, CHA catalysts obtained from sources thatdo not utilize organically template hydrothermal synthesis.

SUMMARY

Aspects of the present invention are directed to zeolites that have theCHA crystal structure (as defined by the International ZeoliteAssociation), processes for preparing such zeolites, catalytic articlescomprising such zeolites, and exhaust gas treatment systems and methodsincorporating such catalytic articles. The catalytic articles may bepart of an exhaust gas treatment system used to treat exhaust gasstreams, especially those emanating from gasoline or diesel engines.

Accordingly, one aspect of the present invention is directed to acatalytic article comprising a zeolite having the CHA crystal structuredisposed on a substrate operative to reduce NOx, wherein the zeolite isa low silica chabazite and having a low alkali content. The zeolite hasa low silica to alumina ratio, for example, below about 15, morespecifically below about 10, even specifically below about 5. Anysubstrate can be used for disposition of the zeolite, including, forexample, honeycomb substrates, foam substrates, and soot filters.

In some embodiments, the zeolite is a non-synthetic, naturally occurringchabazite, such as Bowie chabazite. In other embodiments, the zeolite isa synthetic chabazite. The zeolite may be modified with one or moremetal cations. Suitable metals include any redox active metal,including, but not limited to, copper, iron, and cobalt. The zeolite canbe modified by any method known in the art, such as ion exchange.

Another aspect of the present invention is directed to an exhaust gastreatment system comprising a catalytic article comprising a zeolitehaving the CHA crystal structure disposed on a substrate operative toreduce NOx, wherein the zeolite is a low silica chabazite. In variousembodiments, the catalytic article is in fluid communication with othergas treatment components, such as an oxidation catalyst, a SCR catalyst,an AMOX catalyst, a soot filter, etc.

Another aspect of the present invention is directed to process forreducing NOx in a gas stream comprising contacting the gas stream with acatalytic article of claim 1 article comprising a zeolite having the CHAcrystal structure disposed on a substrate operative to reduce NOx,wherein the zeolite is a low silica chabazite. In some embodiments, thegas stream is contacted with the exhaust article in the presence of NH₃,thereby providing a selective catalytic reduction (SCR) system.

Another aspect of the present invention is directed to a process forpreparing a catalytic article comprising applying a zeolite having theCHA crystal structure as a washcoat to a substrate, wherein the zeoliteis a low silica chabazite and has a low alkali content as describedherein. In some embodiments, the washcoat further comprises a binder. Instill other embodiments, the washcoat further comprises a refractorymetal oxide support. A platinum group metal component may be disposed onthe refractory metal oxide support for further catalytic function.

In some embodiments, the zeolite of the catalytic article is provided bya process comprising mixing an alumina source, a silica source, andsources of one or more of sodium, potassium and TMA to form an aqueousgel, and crystallizing the gel by heating to form the zeolite. Inspecific embodiments, the process is performed at a temperature belowabout 100° C. at atmospheric pressure. In other embodiments, the zeolitemay be provided using an organically templated hydrothermal synthesisprocess in the presence of e.g., KOH, NaOH and tetramethylammoniumhydroxide. In one or more embodiments, the zeolite of the catalyticarticle is mixed with a second zeolite such as zeolite having the CHAcrystal structure and a having a silica to alumina mole ratio exceeding15 as described further below.

Another aspect of the invention pertains to methods of making catalystsof type described above, comprising ion exchanging an alkali form of azeolite having the CHA crystal structure containing an initial alkalicontent with a solution to reduce the alkali content; calcining the ionexchanged zeolite with the reduced alkali content to provide a calcinedzeolite; and subsequently ion exchanging the calcined zeolite with asolution to further reduce the alkali content to provide the zeolitehaving a mole ratio of silica to alumina of less than about 15 and analkali content of less than about 3 weight percent. In specificembodiments, the solution is an ammonium salt solution, and thecalcining occurs at a temperature of at least about 350° C. for at leastabout one hour. In one or more embodiments, methods may further compriseconducting a metal ion exchange with an iron or copper solution toprovide a metal promoted zeolite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph depicting normalized data for NOx conversion (%)versus temperature for fresh and aged CuSynCHA SCR catalytic articlesmade in accordance with the present invention compared to a Fe-Beta SCRcatalytic article;

FIG. 2 is graph depicting NOx conversion (%) versus temperature forfresh and aged CuNatCHA SCR catalytic articles made in accordance withembodiments of the present invention compared to fresh and aged Fe-BetaSCR catalytic articles;

FIG. 3 is graph depicting NOx conversion and N₂O make versus temperaturefor fresh and aged CuNatCHA SCR catalytic articles made in accordancewith embodiments of the present invention; and

FIGS. 4A-C depict various exhaust gas treat systems comprising catalyticarticles made in accordance with the present invention.

DETAILED DESCRIPTION

Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways.

Aspects of the present invention are directed to CHA zeolites and theiruse in catalytic articles, such as SCR catalysts. Such catalyticarticles find particular utility in the treatment of exhaust gasstreams, especially those emanating from gasoline or diesel engines.Applicants have found that catalytic articles made by the processesdisclosed herein exhibit excellent hydrothermal stability and highcatalytic activity over a wide temperature range. When compared withother zeolitic catalysts that find application in this field, such asFe-Beta zeolites, CHA catalytic materials according to embodiments ofthe present invention offer improved low temperature activity andhydrothermal stability. In addition, the CHA catalytic materialsdisclosed herein can be prepared in a more affordable and economicalmanner than SSZ-13 CHA catalysts. Previously, it was believed that lowSi/Al ratio CHA zeolites did not exhibit sufficient NOx conversionand/or hydrothermal stability to be used as NOx reduction catalysts.According to one or more embodiments of the invention, low Si/Al ratioCHA zeolites having low alkali content exhibit good hydrothermalstability and NOx conversion exceeding at least about 50%.

Zeolites

The zeolites used to prepare the catalytic materials disclosed hereinhave the CHA crystal structure. In specific embodiments, the Si/Al ratioof the CHA zeolite is low, more particularly lower than that for SSZ-13CHA. The Si/Al ratio of the CHA zeolite is specifically less than about15, more specifically less than about 10, even more specifically lessthan about 5. Applicants have found that such CHA zeolites can bequickly and economically obtained from natural sources or syntheticallyprepared using processes distinct from those used to prepare CHAzeolites with high Si/Al ratios, such as SSZ-13 CHA. In one or moreembodiments, the CHA zeolite has a low Si/Al ratio and also has a lowalkali content. As used herein, alkali content is expressed on a wt %basis in terms of the respective oxides for sodium, calcium andpotassium present as cations in the zeolite. In one or more embodiments,such CHA zeolites exhibit good hydrothermal stability and NOx conversionfor use as NOx reduction catalyst.

Accordingly, in some embodiments, the CHA zeolite is a non-synthetic,naturally occurring chabazite. Any natural chabazite can be used. Oneparticularly useful form is Bowie chabazite. In some embodiments, thenatural chabazite is purified prior to processing. In other embodiments,the natural chabazite is used unpurified, which may offer catalyticbenefits as targets for HC or S poisoning, and therefore protect thecatalytic articles from deactivation.

As an alternative to natural chabazite, the zeolite can be a syntheticCHA. In specific embodiments, the process used to prepare the syntheticCHA zeolite is a process that does not utilize organically templatedhydrotheinial synthesis used to prepare high silica CHA zeolites, suchas SSZ-13 CHA. The synthetic form of chabazite can be prepared by aprocess comprising mixing an alumina source, a silica source, andsources of one or more of sodium, potassium and trimethylammonium ion(TMA) to form an aqueous gel, and crystallizing the gel by heating toform the zeolite. Typical silica sources include various types of fumedsilica, precipitated silica, and colloidal silica, as well as siliconalkoxides. Typical alumina sources include boehmites, pseudo-boehmites,aluminum hydroxides, aluminum salts such as aluminum sulfate, andaluminum alkoxides. Sodium hydroxide and/or potassium hydroxide may beadded to the reaction mixture, but is not required. In specificembodiments, the process is performed at a temperature below about 100°C. at atmospheric pressure. However, it has been found that suitablesynthetic CHA having the specific Si/Al ratio can be obtained usingorganically templated hydrothermal synthesis in the presence of e.g.,KOH, NaOH and tetramethylammonium hydroxide.

At the conclusion of the synthesis reaction, the CHA product isgenerally filtered and washed with water. Alternatively, the product maybe centrifuged. Organic additives may be used to help with the handlingand isolation of the solid product. Spray-drying is an optional step inthe processing of the product. The solid product is generally thermallytreated in air or nitrogen. Alternatively, each gas treatment can beapplied in various sequences, or mixtures of gases can be applied. Theproduct may be calcined. Typical calcination temperatures are in the400° C. to 700° C. range.

The natural or synthetic chabazite can be modified with one or moremetal cations. Suitable metals include any redox active metal,including, but not limited to, copper, iron, and cobalt. One specificform of modified chabazite is Cu-CHA. Cu-modified natural CHA is hereintermed “CuNatCHA,” while Cu-modified synthetic CHA is herein termed“CuSynCHA.” The zeolite can be metal modified by any method known in theart, such as ion exchange.

In one aspect of the invention, CuNatCHA and CuSynCHA having a low Si/Alratio are provided that also have a relatively low alkali content. Inone embodiment, the alkali content is present in amount less than about6 weight percent, more specifically less than about 3 weight percent,less than about 2 weight percent, less than about 1 weight percent, lessthan about 0.5 weight percent, less than about 0.1 weight percent, orless than 0.05 weight percent. The alkali content can be reduced to thedesired level by a variety of procedures. In an exemplary embodiment, analkali form of the zeolite can be ion exchanged followed by calcinationfollowed by a subsequent ion exchange. As used herein, “alkali form” ofthe zeolite refers to a zeolite containing an alkali metal content suchas Na or K. In specific embodiments, multiple ion exchanges, forexample, two, three, four or five exchanges, occur before calcination.In specific embodiments, ion exchange is performed using an ammoniumsalt such as ammonium nitrate, to provide an ammonium form of thezeolite. Other suitable salts include, but are not limited to ammoniumacetate, carbonate, chloride, citrate, sulfate, and combinations ofthereof. In specific embodiments, calcination is performed at atemperature of at least about 350° C., 400° C., at least about 450° C.,at least about 500° C., or at about 540° C. for at least about 1, 2, 3,or 4 hours. Higher temperatures may be utilized for the calcinationsafter ammonium ion exchange, for example 600° C., 650° C., 700° C. andtemperatures up to about 1000° C. In a specific embodiment, calcinationsis performed after a single ion exchange with ammonium nitrate at 540°C. for at least two hours, and at least two subsequent ion exchanges areperformed to provide a low alkali content zeolite. It has been foundthat a sequence of multiple ion exchanges, followed by calcinations,followed by at least one ion exchange after calcinations significantlyreduces the alkali content of the zeolite. In specific embodiments, thealkali content is reduced to less than about 6, 5, 3, 1, 0.5, 0.1 or0.01 weight percent. Reductions in the alkali oxide weight percent inthe zeolite provide a CuNatCHA and CuSynCHA with a low Si/Al ratio andlow alkali content that exhibit reduction in NOx in exhaust gas streamsexceeding at least about 50%.

In one or more embodiments, the catalytic article comprises CuNatCHA orCuSynCHA having a low Si/Al ratio (e.g., less than 15 or less than 10)are provided that also have a relatively low alkali content (e.g., lessthan about 6 weight percent, 3 weight percent, 1 weight percent or 0.5weight percent, mixed with a zeolite having the CHA crystal structure asilica to alumina ratio greater than 15, an example of which beingSSZ-13. The mixture can contain any suitable amount of each zeolite toobtain the desired NOx reduction and hydrothermal stability. Forexample, the mixture can contain up to 10%, 20%, 30%, 40%, 50%, 60%,70%, 80%, or 90% by weight of a zeolite having the CHA crystal structurea silica to alumina ratio greater than about 15. Desirably, the zeolitecan be promoted with a metal such as iron or copper. The mixture ofzeolites can be achieved by forming separate slurries of the two zeolitepowders and mixing the slurries, or by forming a single slurry of thetwo powders. A particularly suitable zeolite having the CHA crystalstructure has a mole ratio of silica to alumina greater than about 15and an atomic ratio of copper to aluminum exceeding about 0.25. Moreparticularly, the mole ratio of silica to alumina is from about 15 toabout 256, for example 15 to 40 or about 30, and the atomic ratio ofcopper to aluminum is from about 0.25 to about 0.50, more particularlyabout 0.30 to about 0.50, for example 0.40. Examples of such zeolitesare disclosed in U.S. Pat. No. 7,601,662, the entire content of which isincorporated herein by reference.

Experimentation has indicated that improved performance of catalyticarticles in accordance with embodiments of the invention is associatedwith Cu loading. While Cu can be exchanged to increase the level of Cuassociated with the exchange sites in the structure of the zeolite, ithas been found that it is beneficial to leave non-exchanged Cu in saltform, for example, as CuSO₄ within the zeolite catalyst. Uponcalcination, the copper salt decomposes to what is herein known as “freecopper” or “soluble copper.” According to one or more embodiments, thisfree copper is both active and selective, resulting in low N₂O formationwhen used in the treatment of a gas stream containing NOx. Unexpectedly,this free copper has been found to impart greater stability in catalystssubjected to thermal aging at temperatures up to about 800° C.

Substrates

According to some embodiments of the present invention, the CHA zeolitescan be in the form of self supporting catalytic particles. Specifically,however, the CHA zeolite catalysts are disposed on a substrate toprovide a catalytic article. The substrate may be any of those materialstypically used for preparing catalysts, and will usually comprise aceramic or metal honeycomb structure. Any suitable substrate may beemployed, such as a monolithic substrate of the type having fine,parallel gas flow passages extending therethrough from an inlet or anoutlet face of the substrate, such that passages are open to fluid flowtherethrough (referred to as honeycomb flow through substrates). Thepassages, which are essentially straight paths from their fluid inlet totheir fluid outlet, are defined by walls on which the catalytic materialis disposed as a washcoat so that the gases flowing through the passagescontact the catalytic material. The flow passages of the monolithicsubstrate are thin-walled channels, which can be of any suitablecross-sectional shape and size such as trapezoidal, rectangular, square,sinusoidal, hexagonal, oval, circular, etc. Such structures may containfrom about 60 to about 400 or more gas inlet openings (i.e., cells) persquare inch of cross section.

The substrate can also be a wall-flow filter substrate, where thechannels are alternately blocked, allowing a gaseous stream entering thechannels from one direction (inlet direction), to flow through thechannel walls and exit from the channels from the other direction(outlet direction). If a wall flow substrate is utilized, the resultingsystem will be able to remove particulate matter along with gaseouspollutants. The wall-flow filter substrate can be made from materialscommonly known in the art, such as cordierite, aluminum titanate orsilicon carbide. It will be understood that the loading of the catalyticcomposition on a wall flow substrate will depend on substrate propertiessuch as porosity and wall thickness, and typically will be lower thanloading on a flow through substrate.

The ceramic substrate may be made of any suitable refractory material,e.g., cordierite, cordierite-alumina, silicon nitride, zircon mullite,spodumene, alumina-silica magnesia, zircon silicate, sillimanite, amagnesium silicate, zircon, petalite, alpha-alumina, an aluminosilicateand the like.

Substrates may also be metallic in nature and be composed of one or moremetals or metal alloys. The metallic substrates may be employed invarious shapes such as corrugated sheet or monolithic form. Suitablemetallic supports include the heat resistant metals and metal alloyssuch as titanium and stainless steel as well as other alloys in whichiron is a substantial or major component. Such alloys may contain one ormore of nickel, chromium and/or aluminum, and the total amount of thesemetals may advantageously comprise at least 15 wt. % of the alloy, e.g.,10-25 wt. % of chromium, 3-8 wt. % of aluminum and up to 20 wt. % ofnickel. The alloys may also contain small or trace amounts of one ormore other metals such as manganese, copper, vanadium, titanium and thelike. The surface or the metal substrates may be oxidized at hightemperatures, e.g., 1000° C. and higher, to improve the resistance tocorrosion of the alloys by forming an oxide layer on the surfaces thesubstrates. Such high temperature-induced oxidation may enhance theadherence of the refractory metal oxide support and catalyticallypromoting metal components to the substrate.

In alternative embodiments, the CHA zeolite catalysts may be depositedon an open cell foam substrate. Such substrates are well known in theart, and are typically formed of refractory ceramic or metallicmaterials. The catalysts may also be deposited on a soot filter toprepare a catalyzed soot filter (CSF).

Washcoat Preparation

The catalytic articles of embodiments of the present invention aregenerally prepared by applying the CHA zeolite as a washcoat to asubstrate. Washcoats of the CHA zeolite can be prepared using a binder.According to one or more embodiments, a ZrO₂ binder derived from asuitable precursor, such as zirconyl acetate or any other suitablezirconium precursor, such as zirconyl nitrate, is used. In oneembodiment, zirconyl acetate binder provides a catalytic coating thatremains homogeneous and intact after thermal aging, for example, whenthe catalyst is exposed to high temperatures of at least about 600° C.,for example, about 800° C. and higher, and high water vapor environmentsof about 10% or more. Keeping the washcoat intact is beneficial becauseloose or free coating could plug a downstream CSF causing thebackpressure to increase.

In one or more embodiments of the present invention, the CHA zeolitecatalyst comprises a precious metal component, i.e., a platinum groupmetal component. For example, as discussed below, ammonia oxidation(AMOX) catalysts typically include a platinum metal component. Suitableplatinum metal components include platinum, palladium, rhodium andmixtures thereof. The several components (for example, the CHA zeoliteand precious metal component) of the catalyst material may be applied tothe refractory carrier member, i.e., the substrate, as a washcoatmixture of two or more components or as individual washcoat componentsin sequential steps in a manner which will be readily apparent to thoseskilled in the art of catalyst manufacture. This may be accomplished byimpregnating a fine particulate refractory metal oxide support material,e.g., gamma alumina, with one or more catalytic metal components such asa precious metal, i.e., platinum group metal or other noble metals orbase metals, drying and calcining the impregnated support particles, andforming an aqueous slurry of these particles. Particles of the CHAzeolite may be included in the slurry. Activated alumina may bethermally stabilized before the catalytic components are dispersedthereon, as is well known in the art, by impregnating it with, e.g., asolution of a soluble salt of barium, lanthanum, zirconium, rare earthmetal or other suitable stabilizer precursor, and thereafter drying(e.g., at 110° C. for one hour) and calcining (e.g., at 550° C. for onehour) the impregnated activated alumina to form a stabilizing metaloxide dispersed onto the alumina. Base metal catalysts may optionallyalso be impregnated into the activated alumina, for example, byimpregnating a solution of a base metal nitrate into the aluminaparticles and calcining to provide a base metal oxide dispersed in thealumina particles.

The substrate may then be immersed into the slurry of CHA zeolite,binder, and impregnated activated alumina or other support particle (orunsupported precious metal) and excess slurry removed to provide a thincoating of the slurry on the walls of the gas-flow passages of thesubstrate. Alternatively, the supported or unsupported precious metaland CHA zeolite are applied as separate slurries. When deposited on ahoneycomb monolith substrate, the catalytic compositions are generallydeposited at a concentration of at least about 0.5 g/in³, for example,about 1.3 g/in³ about 2.4 g/in³ or higher, to ensure that the desiredNOx reduction is achieved and to secure adequate durability of thecatalytic article over extended use. The coated substrate is then driedand generally calcined to provide an adherent coating of the catalyticmaterial, to the walls of the passages thereof. One or more additionallayers of the slurry may be provided to the carrier. After each layer isapplied, or after a number of desired layers are applied, the carrier isthen dried and calcined to provide a finished catalytic article inaccordance with embodiments of the present invention.

Catalytic Articles

The catalytic articles of the present invention find particular utilityin the treatment of exhaust gas streams, especially those emanating fromgasoline or diesel engines. In use, an exhaust gas stream is contactedwith a catalytic article prepared in accordance with embodiments of thepresent invention. As discussed below, the catalytic articles haveexcellent NOx reduction activity over a wide range of operativetemperatures. As such, the catalytic articles are useful as SCRcatalysts. The term “SCR catalyst” is used herein in a broad sense tomean a selective catalytic reduction in which a catalyzed reaction ofnitrogen oxides with a reductant occurs to reduce the nitrogen oxides.“Reductant” or “reducing agent” is also broadly used herein to mean anychemical or compound tending to reduce NOx at elevated temperature. Inspecific embodiments, the reducing agent is ammonia, specifically anammonia precursor, i.e., urea, and the SCR is a nitrogen reductant SCR.However, in accordance with a broader scope of the invention, thereductant could include fuel, particularly diesel fuel and fractionsthereof as well any hydrocarbon and oxygenated hydrocarbons collectivelyreferred to as an HC reductant.

Theoretically, it would be desirable in the SCR process to provide thereductant (i.e., ammonia) in excess of the stoichiometric amountrequired to react completely with the NOx present, both to favor drivingthe reaction to completion and to help overcome inadequate mixing of theammonia in the gaseous stream. However, in practice, significant excessammonia over such stoichiometric amount is normally not provided becausethe discharge of unreacted ammonia from the catalyst to the atmospherewould itself engender an air pollution problem. Such discharge ofunreacted ammonia can occur even in cases where ammonia is present onlyin a stoichiometric or sub-stoichiometric amount, as a result ofincomplete reaction and/or poor mixing of the ammonia in the gaseousstream, resulting in the formation therein of channels of high ammoniaconcentration. Such channeling is of particular concern when utilizingcatalysts comprising monolithic honeycomb-type substrates comprisingrefractory bodies having a plurality of fine, parallel gas flow pathsextending therethrough because, unlike the case of beds of particulatecatalyst, there is no opportunity for gas mixing between channels.

To address this concern, AMOX catalysts can be provided in the catalyticsystem to oxidize the unreacted ammonia. Applicants have found that aCuCHA washcoat containing a precious metal, for example, Pt, provides anAMOX catalyst. It is expected that not only is ammonia in gas flowingthrough the catalyst destroyed, but there is also continued removal ofNOx by conversion to N₂.

According to methods known in the art, the CHA catalytic articles can beformulated to favor either the SCR process or the AMOX process bycontrolling the Cu content of the zeolite (when present). U.S. Pat. No.5,516,497 teaches iron and copper loading levels on zeolites to obtainselectivity for either an SCR reaction or the oxidation of ammonia byoxygen at the expense of the SCR process, thereby improving ammoniaremoval. In accordance with embodiments of the invention, CuNatCHA orCuSynCHA copper loading can be tailored to obtain selectivity for SCRreactions and oxidation of ammonia by oxygen and to provide exhaust gastreatment systems utilizing both types of catalyst.

Further to this goal, a staged or two-zone catalytic article can beprovided with a first catalytic zone comprising a CHA zeolite thatpromotes SCR followed by a second catalytic zone comprising a CuCHAzeolite that promotes oxidation of ammonia. The resultant catalyticarticle thus has a first (upstream) zone which favors the reduction ofnitrogen oxides with ammonia, and a second (downstream) zone whichfavors the oxidation of ammonia. In this way, when ammonia is present inexcess of the stoichiometric amount, whether throughout the flow crosssection of the gaseous stream being treated or in localized channels ofhigh ammonia concentration, the oxidation of residual ammonia by oxygenis favored by the downstream or second catalyst zone. The quantity ofammonia in the gaseous stream discharged from the catalyst is therebyreduced or eliminated. The first zone and the second zones can beprovided on a single catalytic article or as separate catalyticarticles.

Exhaust Gas Treatment Systems

The CHA zeolite catalytic articles of the present invention can beprovided in an exhaust gas treatment system, such as the ones found ingasoline- and diesel-powered automobiles. In such exhaust gas treatmentsystems, the CHA zeolite catalytic articles generally provided in fluidcommunication with other gas treatment components, either upstream ordownstream of the catalytic articles.

One embodiment of the inventive emissions treatment system denoted as11A is schematically depicted in FIG. 4A. The exhaust, containinggaseous pollutants (including unburned hydrocarbons, carbon monoxide andNOx) and particulate matter, is conveyed from the engine 19 to aposition downstream in the exhaust system where a reductant, i.e.,ammonia or an ammonia-precursor, is added to the exhaust stream. Thereductant is injected as a spray via a nozzle (not shown) into theexhaust stream. Aqueous urea shown on one line 25 can serve as theammonia precursor which can be mixed with air on another line 26 in amixing station 24. Valve 23 can be used to meter precise amounts ofaqueous urea which are converted in the exhaust stream to ammonia.

The exhaust stream with the added ammonia is conveyed to the SCRcatalyst substrate 12 (also referred to herein including the claims as“the first article” or “the first substrate”) containing CuNatCHA orCuSynCHA in accordance with one or more embodiments. On passing throughthe first substrate 12, the NOx component of the exhaust stream isconverted through the selective catalytic reduction of NOx with NH₃ toN₂ and H₂O. In addition, excess NH₃ that emerges from the inlet zone canbe converted through oxidation by a downstream ammonia oxidationcatalyst (not shown) also containing CuNatCHA or CuSynCHA to convert theammonia to N₂ and H₂O. The first substrate is typically a flow throughmonolith substrate.

An alternative embodiment of the emissions treatment system, denoted as11B is depicted in FIG. 4B which contains a second substrate 27interposed between the NH₃ injector and the first substrate 12. In thisembodiment, the second substrate is coated with an SCR catalystcomposition which may be the same composition as is used to coat thefirst substrate 12 or a different composition. An advantageous featureof this embodiment is that the SCR catalyst compositions that are usedto coat the substrate can be selected to optimize NOx conversion for theoperating conditions characteristic of that site along the exhaustsystem. For example, the second substrate can be coated with an SCRcatalyst composition that is better suited for higher operatingtemperatures experienced in upstream segments of the exhaust system,while another SCR composition can be used to coat the first substrate(i.e., the inlet zone of the first substrate) that is better suited tocooler exhaust temperature which are experienced in downstream segmentsof the exhaust system.

In the embodiment depicted in FIG. 4B, the second substrate 27 caneither be a honeycomb flow through substrate, an open cell foamsubstrate or a honeycomb wall flow substrate. In configurations of thisembodiment where the second substrate is a wall flow substrate or a highefficiency open cell foam filter, the system can remove greater than 80%of the particulate matter including the soot fraction and the SOF. AnSCR-coated wall flow substrate and its utility in the reduction of NOxand particulate matter have been described, for instance, in co-pendingU.S. patent application Ser. No. 10/634,659, filed Aug. 5, 2003, thedisclosure of which is hereby incorporated by reference.

In some applications it may be advantageous to include an oxidationcatalyst upstream of the site of ammonia/ammonia precursor injection.For instance, in the embodiment depicted in FIG. 4C an oxidationcatalyst is disposed on a catalyst substrate 34. The emissions treatmentsystem 11C is provided with the first substrate 12 and optionallyincludes a second substrate 27. In this embodiment, the exhaust streamis first conveyed to the catalyst substrate 34 where at least some ofthe gaseous hydrocarbons, CO and particulate matter are combusted toinnocuous components. In addition, a significant fraction of the NO ofthe NOx component of the exhaust is converted to NO₂. Higher proportionsof NO₂ in the NOx component facilitate the reduction of NOx to N₂ andH₂O on the SCR catalyst(s) located downstream. It will be appreciatedthat in the embodiment shown in FIG. 10C, the first substrate 12 couldbe a catalyzed soot filter, and the SCR catalyst could be disposed onthe catalyzed soot filter. In an alternative embodiment, the secondsubstrate 27 comprising an SCR catalyst may be located upstream fromcatalyst substrate 34.

In systems that utilize an SCR catalytic article downstream of a dieseloxidation catalyst (DOC), the properties of the CHA zeolite catalystdisclosed herein may provide one or more beneficial results. Duringstart-up and prolonged low temperature operation, the DOC or DOC and CSFupstream of a SCR catalyst are not fully activated to oxidize thehydrocarbons. Because the CHA zeolite SCR catalytic articles providedherein are not influenced by hydrocarbons at low temperature, it remainsactive over a wider range of the low temperature operation window. Forexample, and as discussed below, CuNatCHA and CuSynCHA catalyticarticles display substantial NOx conversion at temperatures of 250° C.and below. Also, since oxidation catalysts lose their ability to oxidizeNO to NO₂ over time, it is beneficial to provide an SCR catalyst thatcan treat NO as effectively as NO₂. As discussed below, both CuNatCHAand CuSynCHA catalytic articles are capable of reducing NO with NH₃,even at low temperatures.

Without intending to limit the invention in any manner, embodiments ofthe present invention will be more fully described by the followingexamples.

EXAMPLE 1

A K-Chabazite was prepared from an Al₂O₃:5.2 SiO₂:2 K₂O: 224 H₂O gelcomposition. CBV500 (a Zeolite-Y product available from ZeolystInternational, Conshohocken, Pa.) was first calcined in air at 540° C.for 4 hours to prepare the H-form. A reaction mixture was formed bycombining 125 g of this calcined zeolite (H-CBV500) with a mixture of134 g potassium hydroxide (45%) in 991 g of deionized water. Theresulting gel was heated in a 2 liter autoclave at 95° C. for 6 days.The mixture was continuously stirred at 32 rpm. The crystalline productwas recovered via filtration and was washed to a conductivity of 200 μS.The sample was dried at 90° C. The crystalline product had an X-raypowder diffraction pattern indicating that it was Chabazite.

An NH₄ ⁺-form of Chabazite was prepared by exchanging 100 g of theK-Chabazite in a solution of ammonium nitrate (500 g 54 wt % ammoniumnitrate mixed with 500 g of deionized water). The exchange was carriedout by agitating the sluny at 80° C. for 1 hour, during which the pH wasbetween 2.57 and 3.2. The solid was then filtered on a Buchner filterand washed until the filtrate had a conductivity lower than 200 μS. Thepowder was then dried for 16 hours before carrying out the aboveammonium exchange process for a total of two exchanges. By XRF chemicalanalysis, the composition of the solids product was established to be22.86 wt % Al₂O₃, 73.62 wt % SiO₂, and 3.52 wt % K₂O. The SiO₂:Al₂O₃ wascalculated to be 5.5.

The NH₄ ⁺-form was then calcined at 540° C. for 4 hours before carryingout the above ammonium exchange process another 2 times. By XRF chemicalanalysis, the composition of the solids product was established to be23.69 wt % Al₂O₃, 76.25 wt % SiO₂, and 0.08 wt % K₂O. The SiO₂:Al₂O₃ wascalculated to be 5.5.

A CuSynCHA powder catalyst was prepared by mixing 10 g of NH₄ ⁺-formChabazite, with 37 ml of a copper(II) sulfate solution of 1.0 M. Anion-exchange reaction between the NH4⁺-form Chabazite and the copperions was carried out by agitating the slurry at 70° C. for 1 hour. ThepH was between 2.9 and 3.2 during the reaction. The resulting mixturewas then filtered and washed until the filtrate had a conductivity of<200 μS, which indicated that substantially no soluble or free copperremained in the sample. The washed sample was then dried at 90° C. Theobtained Cu-Chabazite catalyst comprised CuO at 3.35% by weight, asdetermined by ICP analysis.

EXAMPLE 2

Catalyst performance for Example 1 was evaluated using a microchannelcatalytic reactor containing a bed of approximately 12.6 mm³ ofcatalyst. The flow rate (standard temperature and pressure) of 25 cc/minof reactants (at the concentration shown in Table 1 below) plus 1.25cc/min steam was passed over the bed at various temperatures (200, 250,300, 350, 400, 450 and 500° C.) to determine the reactivity of thecatalyst. Conversion of NO_(x) was determined by 100*(NO_(x) fed−NO_(x)out)/(NO_(x) fed) using a mass spectral analyzer.

TABLE 1 Species Concentration NO_(x) 500 ppm NH3 500 ppm O₂ 10% Hebalance H₂O as % of  5% dry gas flow

Catalysts were aged using 10% steam in air at 700° C. for 50 hours.

FIG. 1 shows normalized data for nitrogen oxides removal efficiency (%)for the fresh and aged CuSynCHA catalyst as a function of temperaturegenerated on a microchannel catalytic reactor. As can be seen from FIG.1 , the CuSynCHA catalyst has improved NOx conversion in comparison tothe standard Fe-Beta technology, and also exhibits high hydrothermalstability.

EXAMPLE 3

Sedimentary Chabazite was obtained from the deposit at Bowie, Ariz. fromGSA Resources of Tuscan, Ariz. This material is high purity naturalChabazite.

An NFI₄ ⁺-form of Chabazite was prepared by exchanging 350 g of theK-Chabazite in a solution of ammonium nitrate (1750 g 54 wt % ammoniumnitrate mixed with 1750 g of deionized water). The exchange was carriedout by agitating the slurry at 80° C. for 1 hour, during which the pHwas between 2.57 and 3.2. The solid was then filtered on a Buchnerfilter and washed until the filtrate had a conductivity lower than 200μS. The powder was then dried for 16 hours before carrying out the aboveammonium exchange process for a total of two exchanges. By XRF chemicalanalysis, the composition of the solids product was established to be19.75 wt % Al₂O₃, 79.85 wt % SiO2, and 0.4 wt % K₂O. The SiO₂:Al₂O₃ wascalculated to be 6.86.

The NH₄ ⁺-form was then calcined at 540° C. for 4 hours before carryingout the above ammonium exchange process another 2 times. By XRF chemicalanalysis, the composition of the solids product was established to be18.92 wt % Al₂O₃, 80.9 wt % SiO₂, and 0.17 wt % K₂O. The SiO₂:Al₂O₃ wascalculated to be 7.26.

A CuNatCHA powder catalyst was prepared by mixing 230 g of NH₄ ⁺-formChabazite, with 860 ml of a copper (II) acetate solution of 0.5 M. Anion-exchange reaction between the NH4⁺-form Chabazite and the copperions was carried out by agitating the slurry at 70° C. for 1 hour. ThepH was between 3.8 and 4.2 during the reaction. The resulting mixturewas then filtered and washed until the filtrate had a conductivity of<200 μS, which indicated that substantially no soluble or free copperremained in the sample. The washed sample was then dried at 90° C. Theobtained CuNatCHA catalyst comprised CuO at 7% by weight, as determinedby ICP analysis.

A CuNatCHA slurry was prepared by mixing 90 g of Cu-Chabazite, asdescribed above, with 215 mL of deionized water. The mixture wasball-milled for 4 hours to obtain a slurry which comprised 90% particlessmaller than 10 μm. 15.8 g of zirconium acetate in dilute acetic acid(containing 30% ZrO₂) was added into the slurry with agitation.

The slurry was coated onto 1″ D×3″ L cellular ceramic cores, having acell density of 400 cpsi (cells per square inch) and a wall thickness of6.5 mil. The coated cores were dried at 110° C. for 3 hours and calcinedat 400° C. for 1 hour. The coating process was repeated once to obtain atarget washcoat loading of 2.4 g/in³.

EXAMPLE 4

Nitrogen oxides selective catalytic reduction (SCR) efficiency andselectivity of a fresh catalyst core was measured by adding a feed gasmixture of 500 ppm of NO, 500 ppm of NH₃, 10% O₂, 5% H₂O, balanced withN₂ to a steady state reactor containing a 1″ D×3″ L catalyst core fromExample 3. The reaction was carried at a space velocity of 80,000 hr⁻¹across a 150° C. to 460° C. temperature range.

Hydrothermal stability of the catalyst was measured by hydrothermalaging of the catalyst core in the presence of 10% H₂O at, 750° C. for 25hours, followed by measurement of the nitrogen oxides SCR efficiency andselectivity by the same process as outlined above for the SCR evaluationon a fresh catalyst core.

FIG. 2 is graph showing the NOx conversion versus temperature for thefresh and aged CuNatCHA catalyst compared to the current state of theart Fe-Beta catalyst. As can be seen from FIG. 2 , the CuSynCHA catalysthas improved low temperature NOx conversion and greater hydrothermalstability in comparison to the standard Fe-Beta technology.

FIG. 3 is graph showing a comparison of the NOx conversion and N₂O makeor formation versus temperature for the fresh and aged CuNatCHAcatalyst. As can be seen from FIG. 3 , the CuSynCHA catalyst has highselectivity for NOx conversion with low production of undesired N₂O.After aging, the maximum N₂O make was 14 ppm at 450° C.

Comparative Example 1

A K-Chabazite was prepared as described in example 1.

An NH₄ ⁺-form of Chabazite was prepared using the same experimentalconditions described in Example 1. That is, exchanging 100 g of theK-Chabazite in a solution of ammonium nitrate (500 g 54 wt % ammoniumnitrate mixed with 500 g of deionized water). The exchange was carriedout by agitating the slurry at 80° C. for 1 hour, during which the pHwas between 2.57 and 3.2. The solid was then filtered on a Buchnerfilter and washed until the filtrate had a conductivity lower than 200μS. The powder was then dried for 16 hours. This ion-exchange wasrepeated to give a total of 6 ammonium exchanges and chemical analysisis recorded after each experiment in Table 1. No intermediatecalcination step occurred between ammonium exchanges. This example showsthat an intermediate calcination is useful to enable the removal ofpotassium to values lower than ˜1.72 wt %.

TABLE 1 Number of ammonium K₂O loading in ion-exchange steps zeolite (wt%) 1 9.92 2 5.83 3 3.96 4 2.89 5 2.39 6 1.72

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the following claims.

What is claimed is:
 1. A method of making a catalytic articlecomprising: ion exchanging an alkali form of a zeolite having the CHAcrystal structure containing an initial alkali content with a solutionto reduce the alkali content; calcining the ion exchanged zeolite withthe reduced alkali content to provide a calcined zeolite; subsequentlyion exchanging the calcined zeolite with a solution to further reducethe alkali content to provide the zeolite having a mole ratio of silicato alumina of less than about 15 and an alkali content of less thanabout 1 weight percent, subsequently, conducting a metal ion exchangewith an iron or copper solution to provide a metal promoted zeolite, andwherein the alkali content is expressed on a weight percent basis interms of the respective oxides for the alkali present as cations in thezeolite.
 2. The method of claim 1, wherein the solution is an ammoniumsalt solution.
 3. The method of claim 2, wherein the calcining occurs ata temperature of at least about 350° C. for at least about one hour. 4.The method of claim 1, wherein the method comprises a sequence ofmultiple ion exchanges to reduce the alkali content, followed by thecalcining step.
 5. The method of claim 1, wherein, after the subsequention-exchanging step, the alkali content is less than 0.1 weight percent.6. The method of claim 1, wherein, after the subsequent ion-exchangingstep, the alkali content is less than 0.05 weight percent.
 7. The methodof claim 1, wherein the process further comprises subsequently mixingthe zeolite with a second zeolite.
 8. The method of claim 7, wherein thesecond zeolite has a CHA crystal structure.
 9. The method of claim 7,wherein the second zeolite has a silica to alumina mole ratio exceeding15.
 10. The method of claim 1, wherein the alkali form of the zeolitehaving the CHA crystal structure containing the initial alkali contentis a potassium form.